1.
(a) Select all the true statements below.
Select all the true statements below.
- An LHCC equation must have constant coefficients.
- Correct! Constant coefficients are one of the defining features of LHCC equations.
- An LHCC equation could contain the independent variable, \(x \text{.}\)
- Incorrect, LHCC equations are linear, meaning they cannot contain non-linear terms like \(y^2 \text{.}\)
- \(\ds\quad y' + 3y = 0 \) is an LHCC equation.
- Correct! This is a first-order linear homogeneous differential equation.
- A non-homogeneous equation has a non-zero free term.
- Correct! If the free term is not zero, the equation is non-homogeneous.
(b) Linear or Nonlinear.
True.
- Incorrect. The term \(y \cdot y' \) makes this equation nonlinear because the function \(y \) and its derivative are multiplied together.
False.
- Incorrect. The term \(y \cdot y' \) makes this equation nonlinear because the function \(y \) and its derivative are multiplied together.
The equation \(y'' + y \cdot y' - 3y = 0 \) is linear.
(c) True-or-False.
- True
- Correct!
- False
- Incorrect.
(d) Building Solutions.
True.
- Correct! By the superposition principle, this combination is also a solution.
False.
- Correct! By the superposition principle, this combination is also a solution.
If \(y_1\) and \(y_2\) are solutions to a second-order LHCC equation, then
\begin{equation*}
y = 5y_1 - 2y_2
\end{equation*}
is also a solution.
(e) Identifying Homogeneous Equations.
Which of the following equations is homogeneous?
- \(\quad y'' + 3y' + 2y = x \)
- Incorrect. The right-hand side is \(x \text{,}\) which makes the equation nonhomogeneous.
- \(\quad y' + e^x = 0 \)
- Incorrect. The term \(e^x \) on the right-hand side makes the equation nonhomogeneous.
- \(\quad y'' + 5y' + 7y = 0 \)
- Correct! The equation is homogeneous because the right-hand side is zero.
- \(\quad y'' + y' + \sin x = 0 \)
- Incorrect. The \(\sin x \) term on the right-hand side makes the equation nonhomogeneous.
(f) Characteristic equation for first-order LHCC.
What is the characteristic equation for \(y' - 5y = 0\text{?}\)
- \(\ds\quad r - 5 = 0\)
- Correct! The characteristic equation is \(r - 5 = 0\text{.}\)
- \(\ds\quad r + 5 = 0\)
- Incorrect. Check the sign of the coefficient of \(y\text{.}\)
- \(\ds\quad r^2 - 5 = 0\)
- Incorrect. The characteristic equation for a first-order LHCC is linear, not quadratic.
- \(\ds\quad 5r - 1 = 0\)
- Incorrect. Make sure to use the correct coefficients from the original equation.
(g) Identify the first-order LHCC equation.
Which of the following is a first-order LHCC equation?
- \(\ds\quad y'' + y' - y = 0\)
- Incorrect. This is a second-order equation.
- \(\ds\quad 3y' + 5y = 0\)
- Correct! This is a first-order linear homogeneous equation with constant coefficients.
- \(\ds\quad 2y + y' = 3\)
- Incorrect. This equation is not homogeneous.
- \(\ds\quad y' + xy = 0\)
- Incorrect. This is not a constant coefficient equation.
(h) The Characteristic Equation.
What is the characteristic equation of the differential equation \(y'' - 5y' + 6y = 0\text{?}\)
- \(\quad\ds r^2 - 6r + 5 = 0\)
- Incorrect. Check the coefficients in the original equation.
- \(\quad\ds r^2 - 5r + 6 = 0\)
- Correct! The characteristic equation is formed by replacing \(y''\) with \(r^2\text{,}\) \(y'\) with \(r\text{,}\) and \(y\) with 1.
- \(\quad\ds r^2 - 5r - 6 = 0\)
- Incorrect. Be careful with the sign of the free term.
(i) Give the general form.
Give the general form of a second-order LHCC equation if the characteristic equation has the solution: \(r = -1 \pm i\text{.}\)
- \(y = C_1 e^{-x} + C_2 e^{ix}\)
- Incorrect. Complex roots require both cosine and sine terms.
- \(y = (C_1 + C_2 x) e^{-x}\)
- Incorrect. This form is used for repeated real roots.
- \(y = C_1 e^{-x} + C_2 e^{x}\)
- Incorrect. This form is used for distinct real roots.
- \(y = e^{-x} (C_1 \cos(x) + C_2 \sin(x))\)
- Correct! This form is used when the roots are complex.
(j) Give the general form.
Give the general form of a second-order LHCC equation if the characteristic equation has the solutions \(r_1 = 1\) and \(r_2 = -1\text{.}\)
- \(y = C_1 e^{x} + C_2 e^{-x}\)
- Correct! This form is used when the characteristic equation has distinct real roots.
- \(y = C_1 e^{x} + C_2 x e^{x}\)
- Incorrect. This form is used for repeated real roots.
- \(y = (C_1 + C_2 x) e^{x}\)
- Incorrect. This form is also used for repeated real roots.
- \(y = e^{x} (C_1 \cos(x) + C_2 \sin(x))\)
- Incorrect. This form is used for complex roots.
(k) Roots of the characteristic equation.
What are the roots of the characteristic equation \(r^2 - 5r + 6 = 0\text{?}\)
- \(r = 2\) and \(r = 3\)
- Correct! The roots are \(r = 2\) and \(r = 3\text{.}\)
- \(r = -2\) and \(r = -3\)
- Incorrect. Check the signs of the roots.
- \(r = 1\) and \(r = 6\)
- Incorrect. Ensure you solve the quadratic equation correctly.
- \(r = 5\) and \(r = 1\)
- Incorrect. Revisit the quadratic formula to solve for the roots.
(l) General solution for second-order LHCC.
What is the general solution for \(y'' - 5y' + 6y = 0\text{?}\)
- \(y = C_1 e^{2x} + C_2 e^{3x}\)
- Correct! The general solution is \(y = C_1 e^{2x} + C_2 e^{3x}\text{.}\)
- \(y = C_1 e^{-2x} + C_2 e^{-3x}\)
- Incorrect. Check the signs of the exponents.
- \(y = C_1 e^{5x} + C_2 e^{x}\)
- Incorrect. Make sure to use the correct roots.
- \(y = C_1 e^{x} + C_2 e^{-x}\)
- Incorrect. Revisit the roots of the characteristic equation.
(m) Classifying Practice.
Select each classification label that applies to the equation
\begin{equation*}
y'' = y' + 6y
\end{equation*}
- Linear
- Correct, each of the terms are linear.
- Homogeneous
- Correct, the free term is zero.
- Constant Coefficients
- Correct, each coefficient is constant.
- LHCC
- Correct!
(n) Classifying Practice.
Select each classification label that applies to the equation
\begin{equation*}
3y''' + y'- \sin(y) = 0
\end{equation*}
- Linear
- Incorrect, \(\sin(y)\) is a nonlinear term.
- Homogeneous
- Technically, only linear equations can be labeled as homogeneous or not. Since the equation is nonlinear, we do not select it.
- Constant Coefficients
- Technically, only linear equations can be labeled as having constant coefficients or not. Since the equation is nonlinear, we do not select it.
- LHCC
- Incorrect.
(o) Classifying Practice.
Select each classification label that applies to the equation
\begin{equation*}
y''- 6 = 0
\end{equation*}
- Linear
- Correct, both terms are linear.
- Homogeneous
- Incorrect, the free term, \(6\text{,}\) is non-zero.
- Constant Coefficients
- Correct, each coefficient is constant.
- LHCC
- Incorrect.
(p) Classifying Practice.
Select each classification label that applies to the equation
\begin{equation*}
\dfrac{d^3y}{dt^3} + k\dfrac{dy}{dt} = ty, \qquad k \text{ is constant}
\end{equation*}
- Linear
- Correct, all terms are linear.
- Homogeneous
- Correct, the free term is zero.
- Constant Coefficients
- Incorrect, the \(y\) term coefficient, \(t\text{,}\) is not constant.
- LHCC
- Incorrect.
(q) Natural Solutions.
Why do exponentials naturally arise as solutions to LHCC equations?
- Because their derivatives preserve the same functional form.
- Correct! Exponentials are eigenfunctions of the derivative operator.
- Because polynomial solutions cannot satisfy these equations.
- Incorrect. Polynomial solutions can satisfy LHCC equations.
- Because sine and cosine functions do not work.
- Incorrect. Sine and cosine functions are also solutions to LHCC equations.
- Because they minimize the characteristic equation.
- Incorrect. Exponentials are not solutions because they minimize the characteristic equation.
(r) What exponential term is in the solution.
What is the fundamental exponential solution for the equation
\begin{equation*}
-2y' - 7y = 0\text{?}
\end{equation*}
- \(\ds\quad e^{-7/2x}\)
- Correct! Solving \(-2r - 7 = 0\) gives \(r = -\dfrac{7}{2}\text{.}\) So \(\ds e^{-7/2x}\) is the exponential term in the solution.
- \(\ds\quad e^{7/2x}\)
- Incorrect. Check the signs when solving the characteristic equation.
- \(\ds\quad e^{-2/7x}\)
- Incorrect. Ensure you are solving the characteristic equation correctly.
- \(\ds\quad e^{7x}\)
- Incorrect. Write down the characteristic equation and solve for \(r\text{.}\)
(s) Structure of the General Solution.
What is the general solution of a second-order LHCC equation if the characteristic equation has unequal real solutions \(r_1\) and \(r_2\text{?}\)
- \(\quad y = c_1 e^{r_1 x} + c_1 e^{r_2 x}\)
- Incorrect. The constants must be different for each term.
- \(\quad y = c_1 e^{r_1 x} + c_2 e^{r_2 x}\)
- Correct! Each fundamental solution is multiplied by an arbitrary constant.
- \(\quad y = e^{r_1 x} + e^{r_2 x}\)
- Incorrect. The general solution includes arbitrary constants.
(t) General Solution from Repeated Roots.
Suppose the characteristic equation has a triple root at \(r = -1\text{.}\) What is the corresponding part of the general solution?
- \(\quad c_1 e^{-x} + c_2 x e^{-x} + c_3 x^2 e^{-x}\)
- Correct! Each repeated root contributes an extra power of \(x\text{.}\)
- \(\quad c_1 e^{-x} + c_2 e^{-2x} + c_3 e^{-3x}\)
- Incorrect. These are distinct roots, not repeated instances of \(r = -1\text{.}\)
- \(\quad c_1 e^{-x} + c_2 x^2 e^{-x} + c_3 x^3 e^{-x}\)
- Incorrect. The powers should stop at \(x^2\) for a triple root.
- \(\quad c_1 + c_2 x + c_3 x^2\)
- This form is correct for a root at \(r = 0\text{,}\) not \(r = -1\text{.}\)
(u) When to Use a Factoring Tool.
Which of the following is the best reason to use a factoring tool or computer algebra system when solving an LHCC equation?
- The characteristic equation is high degree and does not factor easily by hand.
- Correct! Technology saves time and avoids algebraic errors.
- You already know all the roots from memory.
- Then you wouldnβt need to factor at all.
- You want to verify that exponentials are like-terms.
- Thatβs a useful idea, but not the purpose of a factoring tool.
- The roots are all real and distinct.
- That makes constructing the solution easierβbut you still need the roots first.
(v) Interpreting Mixed Roots.
A characteristic equation has the following roots:
\begin{equation*}
r = 0 \ (\text{double}),\quad r = 2 \pm i\text{.}
\end{equation*}
What is the correct form of the general solution?
- \(\quad y = c_1 + c_2 x + e^{2x}(c_3 \cos x + c_4 \sin x)\)
- Correct! The double root at 0 gives \(1\) and \(x\text{,}\) and the complex pair gives the oscillatory exponential term.
- \(\quad y = c_1 + c_2 x + c_3 \cos(2x) + c_4 \sin(2x)\)
- Incorrect. The sine and cosine terms must be multiplied by \(e^{2x}\text{.}\)
- \(\quad y = c_1 e^{0x} + c_2 x e^{0x} + c_3 e^{ix} + c_4 e^{-ix}\)
- While technically valid, this form is not simplified. Use real-valued functions.
- \(\quad y = c_1 e^{x} + c_2 e^{-x} + c_3 e^{2x}\)
- These roots do not match the ones given.
